Wording: Notifications

[althio]

The TM2 "inbox" system should not be labeled "messages" but rather "notifications".

See occurrences in http://tasks.hotosm.org/user/messages and header drop-down user menu

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Messages (2) >> Notifications (2)
logout (extra note: all items should be consistent, all with first-letter capitalized or none)
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Related to #466 where I understood some of the limitations features of TM2 "message" system. ;)

[ethan-nelson]

With #768, there is the potential for project managers to send a message to all contributors. So they aren't necessarily limited to notifications anymore.